The control of a large operating installation, such as an electric power generating plant or the like in which the respective operating units may be physically remote from each other, presents a number of problems that do not arise in more compact installations. For example, each operating unit may be controlled by a number of subsystems, each of which may constitute a separate data processor for performing a specific control function. The respective subsystems associated with an operating unit may be located in a common cabinet, or they may be physically remote from each other.
In the operation of such a distributed control arrangement, data must be collected from the respective subsystems of each operating unit, often asynchronously; the data must be recognized and perhaps processed; and responsive data or control signals must be transmitted to the same subsystem, or to other subsystems of either the same operating unit or of other operating units of the installation, in order to achieve the desired performance. Thus a communication system is required which is capable of handling the interchange and transfer of the various data and control signals.
In general, when two or more subsystems are in communication with each other in a communication system of the type that is found in use with prior art control arrangements, each data processor specifically addresses, (or is addressed by), the data processor with which information is to be exchanged. In order to hold the communication line between them open, the transmitting data processor must interrupt the receiving data processor if the latter is at that moment involved in another task. By its very nature, the interrupt command will conflict with the demands of the receiving data processor. For example, if the receiving unit is operating on a specific routine, the interrupt command will normally cause it to branch off on a subroutine and to leave the original routine unserviced until the communication with the transmitting data processor is complete.
Where multiple communications between data processors occur, as is the case in a distributed control arrangement for a large installation in which a relatively large number of subsystems is employed, the number of interrupt commands that must be issued is substantial. This may have important effects on the operation of the distributed control system, which may show up as a reduction in the speed of response. While the reduced speed may not be important in the operation of an electric power generating plant, the effect may be significant in other applications where the control system is critically dependent on a rapid response.
A characteristic feature of a prior art communication system in which interrupt orders must be issued to gain access to a receiving data processing system which may be engaged in another operation, is the requirement for setting up levels of priority, i.e. pre-established conditions under which interrupt orders will be honored. In a system where a large number of data processors are required to communicate with each other, as many as fifty separate priority levels may have to be established for this purpose. These priority levels, together with the required branch routines that must be established to enable the receiving data processor to hold its ongoing operation in abeyance pending the completion of the communication with the transmitting unit, serve to make the communication system considerably more complex to build, to operate and to maintain than is the case where interrupt commands do not have to be taken into account.
Another factor that tends to raise the level of complexity in prior art communication systems of the type under discussion, stems from the necessity of two uncoordinated data processors i.e. two uncoordinated subsystems, having to communicate with each other. This entails not only the requirement to subordinate the operation of the receiving data processor to that of the transmitting unit, (which may be the slower unit), but it also requires each processor to be familiar with certain information concerning the unit with which it is in communication. For example, a block of data stored at a certain address in the transmitting unit, may be written into a different location in the receiving unit, and be stored at yet another address in a third unit. Since there is no common discipline which requires that the same data be identified and stored in the same manner in each data processor, the requirement to cope with these situations increases the complexity of each data processor.
A further problem that may arise in prior art communication systems of the type under discussion, is the problem of obtaining a communication line when one is required. Since, for reasons of cost and complexity, the number of existing lines will generally be less than the number demanded at any given point of the operation, a priority system must be set up to prevent contention among competing subsystems for a line, together with suitable safeguards that will enable a subsystem to retain possession until its communication is complete. Here again, the necessity for the additional priorities and safeguards serve to raise the complexity and the cost of such a communication system.
The complexity of prior art communication systems, as discussed above, is largely responsible for elevating relatively simple system changes to the level of major problems that require undue amounts of time and effort to solve. For example, in a large operating installation it is not unusual to add an operating unit, or to withdraw one from operation. Since each operating unit is controlled by a plurality of subsystems, suitable modifications of the associated communication system are required by such a change. Where the latter system is complex, these modifications may entail changes in the priority levels with respect to interrupt commands. Changes with respect to access to the communication lines of the newly added subsystems may also be required or, if an operating unit is withdrawn, with respect to access to the subsystems that remain. Simply stated, the complexity of prior art communication systems of the type under discussion limits their flexibility to adapt easily to changes of the distributed control arrangement with which such systems are associated.